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To()IWC E r►MI193o67 <br />DEERE & AULT <br />RECEIVED <br />May 14, 2015 MAY 19 2015 <br />DIVISION OF RECLAMATION <br />MINING AND SAFETY <br />Mr. Ben Frei <br />Albert Frei and Sons <br />P.O. Box 700 <br />Henderson, Colorado 80640 <br />Re: Alluvial Underflow Estimate in the vicinity of Stagecoach Reservoir <br />Dear Ben: <br />This letter summarizes our estimate of the approximate magnitude of the alluvial underflow in <br />the South Platte River alluvial aquifer in the vicinity of Stagecoach Reservoir. Our calculation <br />represents the geometry of the alluvial aquifer before construction and lining of any gravel pit <br />reservoirs in the area. We used published hydrogeologic data inventoried in a GIS by the <br />Colorado Decision Support System (CDSS), and USGS Professional Paper 1705 entitled <br />"Stratigraphy, lithology and sedimentary features of Quaternary alluvial deposits of the South <br />Platte River and some of its tributaries east of the Front Range, Colorado " by D.A. Lindsey et <br />al. (2005). The CDSS data includes hydraulic conductivity values from well testing, the <br />configuration of the bottom of the aquifer (bedrock elevation contours) and the altitude of the <br />water table (Figure 1). <br />We used Darcy's Law to calculate the underflow through a representative cross-section of the <br />aquifer. The cross-section line shown in plan view on Figure 1 is roughly the same location as <br />Section M in Lindsey et al. (2005). The equation for Darcy's Law is Q = KiA where, <br />Q = Flow rate (cubic feet per day) <br />K = Hydraulic Conductivity (feet per day) <br />i = Hydraulic Gradient (dimensionless) <br />A = Cross -Sectional Area (square feet) <br />The hydraulic conductivity was estimated to be 800 ft/day based on pumping tests performed on <br />four wells in the general vicinity of the section (Figure 1). Using the positions of the water table <br />elevation contours, the hydraulic gradient was calculated to be 5.3x10-5. The cross-sectional area <br />from the western edge of the alluvial aquifer to U.S. Highway 85 was measured to be about <br />352,000 ft2 (Figure 2). This is the area that currently contains numerous lined gravel pit <br />reservoirs. Performing the calculation yields an alluvial underflow of about 14,800 ft3/day, or <br />approximately 77 gallons per minute. <br />600 S. Airport Road, Building A, Suite 205 <br />Longmont, CO 80503 <br />Phone: 303-651-1468 • Fax: 303-651-1469 <br />